A conventional art will be described with reference to FIG. 12.
Conventionally, a supply and exhaust ventilation device of this kind includes, in main body 101, heat exchanger 103 for exchanging heat. Further, in main body 101, there are provided supply air path 102 via heat exchanger 103, heat exchange air path 104 via heat exchanger 103, ventilation air path 105 not via heat exchanger 103, and damper 106 for switching between heat exchange air path 104 and ventilation air path 105. Further, supply air path 102 is provided with outdoor temperature/humidity sensor 107 for detecting outdoor temperature and outdoor humidity, and an exhaust air path constituted by heat exchange air path 104 and ventilation air path 105 is provided with indoor temperature/humidity sensor 108 for detecting indoor temperature and indoor humidity (see PTL 1, for example).
Such a conventional supply and exhaust ventilation device can be controlled based on indoor and outdoor enthalpies so that the switching between the heat exchange ventilation and the normal ventilation does not cause discomfort to users. However, in some cases, the control is made to perform the heat exchange ventilation even in a condition in which the users do not feel discomfort because of a small difference between the indoor and outdoor enthalpies.